The up/down arrows will zoom the map in and out just like clicking on the ± on the left hand scale slider bar. Most of you probably already know that.

Double-clicking in the scale slider bar will set the zoom to that exact point.

Left/right arrows will increase/decrease the transparency (decrease/increase the saturation) of the OSM map backgrounds. This is pretty handy when there's lots of tiny little stations on the screen that you're trying to see.

When you make the OSM maps 100% transparent (the whole way to the left), APRSISCE/32 switches from the Mercator projection that we're all used to and instead places stations according to their range and bearing from the center of the screen. As of a fairly recent version, it also adds lat/lon lines at 10 degree increments. Dragging the world map in this mode is quite interesting, especially if you have stations visible from all over the world. It makes it pretty obvious that things that appear to be East or West of you are in fact, closer if you go "over the pole".

I know there's a name for this projection (Azimuthal equidistant projection thanks to James VE6SRV) because HF operators use something similar from their QTH to figure out the actual compass setting to point there beam to work certain areas of the world. Mercator projections don't make this easy.